The present application relates to an optical recording medium used as a pattern medium in which a track on which a plurality of small record carriers in which a record state is held by modulation corresponding to irradiation of light are arranged is formed, and recording information is expressed by a record/non-record (or erase) pattern of the small record carrier on the track.
Further, the present application relates to a recording/reproducing apparatus and method which perform recording and reproducing on an optical recording medium used as a pattern medium.
For example, a so-called optical disc recording medium (which may also be referred to simply as an “optical disc”) such as a compact disc (CD), a digital versatile disc (DVD), and a Blu-ray disc (BD) (a registered trademark) has widely been spread as an optical recording medium that records and reproduces information by irradiation of light.
On optical discs, a reduction in a wavelength of recording/reproducing light and an increase in a numerical aperture of an objective lens are being made. Thus, a beam spot size for recording/reproducing is reduced, leading to a high recording capacity and high recording density.
However, in optical discs, air is used as a medium between an objective lens and the optical disc, and it is difficult to increase the numerical aperture NA having influence on the size (diameter) of a focus spot to be larger than “1”.
Specifically, when a numerical aperture of an objective lens is NAobj, and a wavelength of light is λ, the size of a spot of light that irradiates onto an optical disc through an objective lens is expressed as follows:λ/NAobj 
At this time, when a refractive index of a medium interposed between the objective lens and the optical disc is nA, and an incident angle of a light beam around the objective lens is θ, the numerical aperture NAobj is expressed as follows:NAobj=nA×sin θ
As can be seen from this formula, it is difficult to increase the numerical aperture NAobj to be larger than 1 as long as a medium is air (nA=1).
In this regard, recording/reproducing methods (a near field method) that implement NAobj>1 using near-field light (evanescent light) have been proposed as disclosed in Japanese Patent Application Laid-Open (JP-A) No. 2010-33688, Japanese Patent Application Laid-Open (JP-A) No. 2009-134780, and the like.
This near field method is known to record or reproduce information by irradiating an optical disc with near-field light, and a solid immersion lens (hereinafter referred to as a “SIL”) is used as an objective lens used to irradiate the optical disc with near-field light (for example, see JP-A No. 2010-33688 and JP-A No. 2009-134780).
FIG. 17 is a diagram to describe a near field optical system of a related art using an SIL.
FIG. 17 illustrates an example in which an SIL of a super hemispherical shape (super hemispherical SIL) is used as an SIL. Specifically, in the super hemispherical SIL in this case, an object side (that is, a side facing a recording medium which is a recording/reproducing target) has a planar shape, and the other portions have a super hemispherical shape.
In this case, an objective lens is configured as a two-group lens including the super hemispherical SIL as a front lens. As shown in FIG. 17, a double-sided aspherical lens is used as a rear lens.
Here, when an incident angle of incident light is θi, and a refractive index of a constitutional material of the super hemispherical SIL is nSIL, an effective numerical aperture NA of the objective lens having the configuration illustrated in FIG. 17 is expressed as follows:NA=nSIL×sin θi
Through this formula, when the configuration of the objective lens illustrated in FIG. 17 is employed, the effective numerical aperture NA can be larger than “1” by setting the refractive index nSIL of the SIL to be larger than “1” (larger than a refractive index of air).
In the related arts, for example, the refractive index nSIL of the SIL is set at about 2, and thus the effective numerical aperture NA of about 1.8 is implemented.
Here, in the near field optical system, an SIL of a hemispherical shape (hemispherical SIL) as well as the super hemispherical SIL is used.
When the hemispherical SIL is used for the objective lens instead of the super hemispherical SIL illustrated in FIG. 17, an effective numerical aperture NA is as follows:NA=nSIL×sin θi
Through this formula, even when the hemispherical SIL is used, when a high refractive index material of nSIL is used as a constitutional material of an SIL, NA>1 can be implemented.
At this time, compared with the formula in the case of the super hemispherical SIL, when the constitutional material (refractive index) of the SIL is the same in both of the case of the super hemispherical shape and the hemispherical shape, the effective numerical aperture NA in the case of using the super hemispherical SIL is higher.
For the sake of confirmation, in order to perform recording/reproducing propagating (irradiating) light of NA>1 generated by the SIL to a recording medium, it is necessary to arrange an object plane of the SIL and the recording medium to be very close to each other. A distance between an objective surface of the SIL and the recording medium (recording surface) is called a gap.
In the near field method, it is necessary to suppress a gap value to be equal to or less than at least a fourth (¼) a wavelength of light.
Meanwhile, in the related arts, studies on the structure of an optical recording medium have been conducted in order to implement high recording density. For example, the structure of an optical recording medium by a so-called pattern medium has been proposed as disclosed in Japanese Patent Application Laid-Open No. 2006-73087.
Similarly to proposals in a magnetic recording field, a pattern medium is configured such that a track on which small record carriers are arranged is formed, and recording information is expressed by a record/non-record (or erase) pattern of the small record carrier on the track. Specifically, one small record carrier functions as one code (“0” or “1”).
Since the small record carrier is independently formed, even if the small record carriers are arranged to be close to one another, that is, even if the small record carriers are arranged with high density, cross light or crosstalk can be suppressed. In other words, the recording density can be increased accordingly.